Electro-optic liquid crystal camera iris providing angle independent transmission for uniform gray shades

ABSTRACT

A high-contrast electro-optic liquid crystal camera iris provides angle independent transmission for uniform gray shades. The liquid crystal iris comprises a combination of first and second liquid crystal devices arranged in optical series and positioned between optical polarizers. The director field of the second liquid crystal device is a mirror image of the director field of the first liquid crystal device, and the first and second liquid crystal devices are placed together so that the azimuthal directions of the surface-contacting directors are in parallel alignment at the adjoining or confronting surfaces of the substrates of the first and second liquid crystal devices. The liquid crystal iris provides, therefore, less angular variation of intermediate transmittances compared with that provided by prior art liquid crystal iris.

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TECHNICAL FIELD

This disclosure relates to a liquid crystal iris for an image recordingdevice such as a camera and, in particular, for a miniature camera of atype installed in a smart phone.

BACKGROUND INFORMATION

A conventional camera uses a mechanical iris diaphragm to control theamount of light reaching the recording medium, such as film or acharge-coupled device (CCD) light sensor array. The mechanical iris iscomplex device that is ill-suited for many miniature cameraapplications, such as those found in smart phones and other hand-helddevices. Many electro-optical alternatives to the mechanical iris havebeen proposed such as the guest-host liquid crystal display (U.S. Pat.No. 4,774,537 and U.S. Patent Application Pub. No. US 2012/0242924),twisted nematic (TN) liquid crystal display (U.S. Patent ApplicationPub. No. US 2008/0084498), electrophoretic display (U.S. Pat. No.7,859,741), digital micro-mirror display (European Patent ApplicationNo. EP1001619 A1), and electrowetting display (U.S. Pat. No. 7,508,566).All of these alternatives can control the amount of light admitted tothe recording medium, and if the electrode structures are patterned inan arrangement of concentric rings to enable changing the aperture,these alternative approaches can also adjust the depth of field. Such apatterned ring arrangement of electrode structures is described indetail, for example, in U.S. Patent Application Pub. No. US2008/0084498.

Using liquid crystals for an electro-optical iris is particularlyattractive since liquid crystal devices (LCDs) are a mature mainstreamtechnology. As with the mechanical iris, to control light effectively,the liquid crystal iris must be capable of providing not only a highcontrast ratio, but also a uniform transmittance at intermediate graylevels over the range of light input angles that are typically found inminiature camera optics. Angular dependence of the transmittance of theiris would cause a nonuniform exposure over the light-sensitive area ofthe recording medium. Prior art liquid crystal iris designs do notsatisfy these requirements, which shortcomings are reasons why liquidcrystal irises have not yet found widespread commercial application.

Simulations show that a TN device used as a camera iris, as proposed inU.S. Patent Application Pub. No. US 2008/0084498, provides a very highcontrast ratio, but the transmittance at intermediate gray levels isappreciably nonuniform over a range of angles of incident light. FIG. 1shows a simulated electro-optic curve of a prior art TN iris deviceunder conditions of normally incident white light. This curve issimulated by use of Display Modeling System (DIMOS) software availablefrom Autronic-Melchers GmbH, Karlsruhe, Germany, for the liquid crystalmixture MLC-7030 available from Merck GmbH, Darmstadt, Germany. MLC-7030has a positive dielectric constant anisotropy of 3.8 and a birefringenceof Δn=0.1126 at the 550 nm design wavelength. The cell gap is chosen4.23 μm to satisfy the “first minimum” condition Δn·d/λ=0.866 to providemaximum throughput at the design wavelength. Using the data of FIG. 1,the normalized transmitted luminance is 50% at 2.81V, and the normalizedtransmitted luminance is 0.1% at 5.25V, resulting in a contrast ratio of1,000.

FIG. 2 shows, for this simulation, the angular variation of thenormalized transmitted luminance of the prior art TN iris underapplication of a drive voltage of 2.81V for 50% transmitted luminance atnormal incidence. These data are conveniently presented by aniso-transmitted luminance polar contour diagram, in which the contoursare lines of constant normalized transmitted luminance. The center ofthe diagram represents normal incidence, where the normalizedtransmitted luminance is 50% and the periphery of the diagram representsincident light at the polar angle of 40°. The azimuthal angle of theincident light is represented in the circular direction from 0° to 360°.FIG. 2 shows that the normalized transmitted luminance varies from about9% to about 82% over incident angles extending out to 20°. Although thisTN device can achieve a high contrast ratio, the strong angularvariation of intermediate transmittances of this TN device makes itunsatisfactory for use as a camera iris.

The prior art dual-cell guest-host iris represented in FIGS. 6A and 6Bof U.S. Patent Application Pub. No. US 2012/0242924, consists of twohomogeneously aligned guest-host cells placed in optical series andoriented with their surface alignment directions at 90°. FIG. 3 shows asimulated electro-optic curve for this prior art iris with 5 μm cellgaps filled with the liquid crystal mixture MLC-7030, to which is addedan achromatic organic dye mixture with a dichroic ratio of 6.2.Simulations show that, even at 12V, the contrast ratio reaches only 4.1.FIG. 4 shows a simulated iso-transmitted luminance polar contour diagramof the normalized transmitted luminance under application of 3.1V for50% normalized transmitted luminance at normal incidence. In FIG. 4, thenormalized transmitted luminance varies from about 42% to 60% over arange of incident angles extending out to polar angles of 20°. FIG. 4exhibits somewhat less angular variation of intermediate transmittancescompared with the angular variation of the TN iris simulated in FIG. 2,but the low contrast ratio of the dual-cell guest-host iris makes itunsatisfactory for use as a camera iris.

SUMMARY OF THE DISCLOSURE

The disclosed liquid crystal electro-optic iris overcomes the shortfallsof prior art liquid crystal irises by providing a high contrast ratiowith little angular variation of intermediate transmitted luminances.The disclosed liquid crystal iris comprises a combination of first andsecond liquid crystal cells or devices arranged in optical series andpositioned between optical polarizers. The director field of the secondliquid crystal device is a mirror image of the director field of thefirst liquid crystal device, and the two liquid crystal devices areplaced together so that the azimuthal directions of thesurface-contacting directors are in parallel alignment at the adjoiningor confronting surfaces of the substrates of the two liquid crystaldevices. This arrangement of the first and second liquid crystal devicesis notably different from that of prior art dual-cell assemblies, inwhich the azimuthal directions of the surface-contacting directors ofthe adjacent substrates are in orthogonal alignment to each other.

A qualitative explanation for the above-described arrangement of thefirst and second liquid crystal devices of the present invention is asfollows. For example, in the first liquid crystal device, intermediatetransmitted luminances can exhibit considerable angular variation whenthe propagation direction of the light in the liquid crystal layer isapproximately along the direction of the tilted director located in themiddle of the liquid crystal layer. But, as the light passes through thesecond liquid crystal device, the angular variation of the combinedtransmittance is dramatically reduced because the tilted directorlocated in the middle of the second liquid crystal layer is morebroadside to the light propagation direction. Placing the two liquidcrystal devices together in the arrangement described results,therefore, in light that passes through the first liquid crystal devicein a “bad” direction and passes through the second liquid crystal devicein a “good” direction, and vice versa. The disclosed dual-cell liquidcrystal iris provides, therefore, less angular variation of intermediatetransmittances compared with that provided by prior art liquid crystalirises.

In a first embodiment, the liquid crystal devices are non-twisted,electrically controlled birefringence (ECB) cells, in which the nematicliquid crystal can be of a type with positive dielectric constantanisotropy or with negative dielectric constant anisotropy. For the caseof positive dielectric anisotropy, the surface-contacting directors makesmall pretilt angles from the plane of the liquid crystal layer(homogeneous alignment) and, for the case of negative dielectricanisotropy, the surface-contacting directors make small pretilt anglesfrom the direction of the normal to the liquid crystal layer(quasi-homeotropic alignment). The cell gap of each liquid crystaldevice is chosen to provide a continuous, voltage-dependent phase shiftchange up to 90° between the ordinary and extraordinary polarizationcomponents of light passing through the device. The desired phase shiftchange is accomplished by placing the two liquid crystal devicestogether so that the azimuthal directions of the surface-contactingdirectors of the adjoining or confronting surfaces of the substrates ofthe two liquid crystal devices are in parallel alignment. The 90° phaseshift change imparted by each of the liquid crystal devices adds up to a180° phase shift change for the combination, which is equivalent to a90° rotation of linearly polarized light to provide maximum lighttransmittance when the first and second liquid crystal devices areplaced between orthogonally aligned polarizers.

In a second embodiment, the liquid crystal has a positive dielectricanisotropy, and the liquid crystal layer twist angles of the two liquidcrystal devices are substantially 60° but of opposite twist sensebecause the director field of the second liquid crystal device is amirror image of the director field of the first liquid crystal device.Furthermore, in this second embodiment, the product of the cell gap, d,times the birefringence of the liquid crystal, Δn, is approximatelygiven by the formula Δn·d/λ=0.629, where λ is the design wavelength. Thetwo liquid crystal devices are placed together so that the azimuthaldirections of the surface-contacting directors of the adjoining orconfronting surfaces of the substrates of the two liquid crystal devicesare in parallel alignment. In this second embodiment, the inputpolarization direction is set to approximately bisect the angulardistance between azimuthal directions of the surface-contactingdirectors at the inner substrate surfaces of the first liquid crystaldevice.

When no voltage is applied to the liquid crystal devices of the secondembodiment, the light exiting the first liquid crystal device iscircularly polarized at the design wavelength and the light exiting thesecond liquid crystal device is linearly polarized, but rotated by 90°the input linear polarization direction. When the liquid crystal devicesof this arrangement are activated with optimum high drive voltage, thelight exiting the first liquid crystal device is linearly polarized andthe light exiting the second liquid crystal device is linearly polarizedin the same direction as the input linear polarization direction.Placing the second embodiment between orthogonally aligned polarizersprovides maximum light transmittance when no voltage is applied to thefirst and second liquid crystal devices and essentially zerotransmittance when a high voltage is applied to the first and secondliquid crystal devices, thereby assuring maximum throughput and a highcontrast ratio. Intermediate gray levels are obtained by applyingvoltages intermediate between 0V and the high drive voltage.

In a third embodiment, the liquid crystal has a positive dielectricanisotropy, and the liquid crystal layer twist angles of the two liquidcrystal devices are substantially 90° but of opposite twist sensebecause the director field of the second liquid crystal device is amirror image of the director field of the first liquid crystal device.Furthermore, in this third embodiment the product of the cell gap, d,times the birefringence of the liquid crystal, Δn, is approximatelygiven by the formula Δn·d/λ=0.447, where λ is the design wavelength. Thetwo liquid crystal devices are placed together so that the azimuthaldirections of the surface-contacting directors of the adjoining orconfronting surfaces of the substrates of the two liquid crystal devicesare in parallel alignment. In this third embodiment, the inputpolarization direction is set to approximately 20° with respect to theazimuthal direction of the surface-contacting directors at the inputsurface of the first liquid crystal device.

Additional aspects and advantages will be apparent from the followingdetailed description of preferred embodiments, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simulated normalized transmitted luminance electro-opticcurve of a prior art iris comprising a TN liquid crystal device.

FIG. 2 shows a normalized iso-transmitted luminance polar contourdiagram of the prior art camera iris of FIG. 1, comprising a single TNliquid crystal device with a drive voltage adjusted to give 50%normalized transmitted luminance for normally incident light.

FIG. 3 is a simulated normalized transmitted luminance electro-opticcurve of a prior art camera iris comprising two orthogonally alignedguest-host liquid crystal devices.

FIG. 4 shows a simulated normalized iso-transmitted luminance polarcontour diagram of the prior art camera iris of FIG. 3, comprising twoorthogonally aligned guest-host liquid crystal devices with a drivevoltage adjusted to give 50% normalized transmitted luminance fornormally incident light.

FIG. 5 is a diagrammatic exploded view of a first embodiment of thedisclosed electro-optic liquid crystal camera iris comprising first andsecond homogeneously aligned ECB devices, in which the director field ofthe second liquid crystal device is a mirror image of the director fieldof the first liquid crystal device, and in which the first and secondliquid crystal devices are placed together so that the azimuthaldirections of the surface-contacting directors of the adjoining orconfronting surfaces of the substrates of the two liquid crystal devicesare in parallel alignment.

FIG. 6 is a normalized transmitted luminance electro-optic curve of thefirst embodiment of FIG. 5, showing the normalized transmitted luminanceas a function of applied voltage.

FIGS. 7A, 7B, and 7C show normalized iso-transmitted luminance polarcontour diagrams of the camera iris according the first embodimentdepicted in FIG. 5, with the drive voltage adjusted to give,respectively, 75%, 50%, and 25% normalized transmitted luminance fornormally incident light.

FIG. 8A shows a measured normalized transmitted luminance electro-opticcurve according to the first embodiment depicted in FIG. 5.

FIGS. 8B, 8C, and 8D show measured iso-transmitted luminance polarcontour diagrams according to the first embodiment depicted in FIG. 5,taken at, respectively, 1.41V with 75% normalized gray level at normalincidence, 1.74V with 50% normalized gray level at normal incidence, and2.22V with 25% normalized gray level at normal incidence.

FIG. 9 is a simulated normalized transmitted luminance electro-opticcurve of a second embodiment of the disclosed electro-optic liquidcrystal camera iris, showing the normalized transmitted luminance as afunction of applied voltage.

FIG. 10 shows a simulated normalized iso-transmitted luminance polarcontour diagram of the camera iris according the second embodiment, withthe drive voltage adjusted to give 50% normalized transmitted luminancefor normally incident light.

FIG. 11 is a simulated normalized transmitted luminance electro-opticcurve of a third embodiment of the disclosed electro-optic liquidcrystal camera iris, showing the normalized transmitted luminance as afunction of the applied voltage.

FIG. 12 shows the normalized iso-transmitted luminance polar contourdiagram of the camera iris according the third embodiment, with thedrive voltage adjusted to give 50% normalized transmitted luminance fornormally incident light.

FIG. 13 is a block diagram showing one example of a camera moduleimplemented with the disclosed liquid crystal iris.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 5 is a simplified diagram of a liquid crystal iris 10 configured asa first embodiment of the disclosed electro-optic liquid crystal camerairis. Liquid crystal iris 10 comprises a first ECB liquid crystal device12 and a second ECB liquid crystal device 14, each of which containsliquid crystal having a positive dielectric anisotropy. For simplicity,index matching coatings of each of ECB liquid crystal devices 12 and 14are omitted from the diagram. ECB liquid crystal devices 12 and 14 have,respectively, a director field 16 composed of liquid crystal directors18 and a director field 20 composed of liquid crystal directors 22. Eachof director fields 16 and 20 is shown in FIG. 5 at an intermediate drivevoltage, V. Director field 20 of ECB liquid crystal device 14 is amirror image of director field 16 of ECB liquid crystal device 12. Inother words, liquid crystal directors 22 in director field 20 arereversely arranged in comparison to corresponding liquid crystaldirectors 18 in director field 16.

ECB liquid crystal device 12 has a spaced-apart pair of first electrodestructures that include substrate plates 24 ₁ and 24 ₂. An opticallytransparent electrode 26 ₁ formed on substrate plate 24 ₁ constitutes,for one first electrode structure of the pair, an interior surface onwhich is formed an alignment layer 28 ₁. An optically transparentelectrode 26 ₂ formed on substrate plate 24 ₂ constitutes, for the otherfirst electrode structure of the pair, an interior surface on which isformed an alignment layer 28 ₂. Alignment layers 28 ₁ and 28 ₂ haverespective alignment surfaces 30 ₁ and 30 ₂.

ECB liquid crystal device 14 has a spaced-apart pair of second electrodestructures that include substrate plates 32 ₁ and 32 ₂. An opticallytransparent electrode 34 ₁ formed on substrate plate 32 ₁ constitutes,for one second electrode structure of the pair, an interior surface onwhich is formed an alignment layer 36 ₁. An optically transparentelectrode 34 ₂ formed on substrate plate 32 ₂ constitutes, for the othersecond electrode structure of the pair, an interior surface on which isformed an alignment layer 36 ₂. Alignment layers 36 ₁ and 36 ₂ haverespective alignment surfaces 38 ₁ and 38 ₂.

Surface-contacting directors 18 c and 22 c make angles α with theirrespective alignment surfaces 30 ₁, 30 ₂, and 38 ₁, 38 ₂. Azimuthaldirections of the surface-contacting directors are indicated by arrows.Specifically, arrows 40 indicate the azimuthal direction ofsurface-contacting directors 18 c and 22 c at alignment surfaces 30 ₁and 38 ₂, respectively; and arrows 42 indicate the azimuthal directionof surface-contacting directors 18 c and 22 c at alignment surfaces 30 ₂and 38 ₁. Arrows 42 are parallel at the adjoining or confrontingsurfaces of substrate plates 24 ₂ and 32 ₁ of ECB liquid crystal devices12 and 14, respectively. To obtain high contrast ratios, one or moreexternal retarders are included in liquid crystal iris 10 to compensatefor the combined residual retardation of ECB liquid crystal devices 12and 14. The slow axis of the retarder is set in generally perpendicularalignment to the azimuthal direction of the surface-contacting directorsof the alignment layer to which the retarder is in adjacent position,although other retarder axis orientations are also possible. Theretarder or retarders can be placed at locations 44 ₁, 44 ₂, and 44 ₃,as indicated in FIG. 5. ECB liquid crystal devices 12 and 14 arepositioned between linear polarizers 46 and 48, with their transmissionaxes orthogonally aligned. Incoming light 50 is incident on polarizer46.

FIG. 6 shows a simulated electro-optic curve of an example of the firstembodiment represented by liquid crystal iris 10. The liquid crystalMLC-7030 is used in the simulation, and the cell gap of each of ECBliquid crystal devices 12 and 14 is set at 1.50 μm. The pretilt angle αis 3°. To obtain a high contrast ratio, 31.4 nm polycarbonate retardersare placed at locations 44 ₂ and 44 ₃, with their slow axes set inperpendicular alignment to azimuthal direction 40 of thesurface-contacting directors 18 c and 22 c of ECB liquid crystal devices12 and 14, respectively. In this orientation, the two retarders fullycompensate the combined residual retardation of ECB liquid crystaldevices 12 and 14 at an applied voltage of 6.51V. The combination of ECBliquid crystal devices 12 and 14 and associated retarders at locations44 ₂ and 44 ₃ is placed between orthogonally aligned polarizers 46 and48 such that azimuthal directions 40 of surface-contacting directors 18c and 22 c of the respective ECB liquid crystal devices 12 and 14 make a45° angle with the polarization direction of light incident on a lightinput face 52 of substrate plate 24 ₁. The normalized transmittedluminance is 50% for an applied voltage of 2.95V, and the normalizedtransmitted luminances at 6.16V is 0.1%, thereby resulting in a contrastratio of 1,000.

FIGS. 7A, 7B, and 7C show the angular dependence of the normalizedtransmitted luminance under application of three different drivevoltages, V. Drive voltages of 2.55V, 2.95V, and 3.47V, give,respectively, 75%, 50%, and 25% transmitted luminance at normalincidence. As in FIG. 2, these data are presented in FIGS. 7A, 7B, and7C in the form of normalized iso-transmitted luminance polar contourdiagrams. Comparing FIG. 7B with FIG. 2 for the prior art TN iris, it isapparent that there is remarkable improvement in the uniformity of theangular dependence of the 50% gray level. FIGS. 7A and 7C show gooduniformity of angular dependence of the 75% gray level and of the 25%gray level, respectively. Liquid crystal iris 10 of the first embodimentis eminently suitable for use as a camera iris because it can achieve ahigh contrast ratio and exhibits capability to maintain a uniform graylevel over a wide range of light input angles.

FIG. 8A shows a measured normalized transmitted luminance electro-opticcurve at normal incidence for an example of liquid crystal iris 10 ofthe first embodiment. The liquid crystal used in ECB liquid crystaldevices 12 and 14 for this measurement has a birefringence of 0.099 at589 nm and 20° C., and the cell gap of each of ECB liquid crystaldevices 12 and 14 is set at 1.4 μm. A first 15 nm retarder film isplaced in retarder location 44 ₂, at light input face 52 of substrateplate 24 ₁ of ECB liquid crystal device 12. The slow axis of the first15 nm retarder is set perpendicular to azimuthal direction 40 ofsurface-contacting directors 18 c at light input face 52. A second 15 nmretarder film is placed in retarder location 44 ₃, at a light exit face54 of substrate plate 32 ₂ of ECB liquid crystal device 14. The slowaxis of the second 15 nm retarder is set perpendicular to azimuthaldirection 40 of surface-contacting directors 22 c at light exit face 54.The combination of ECB liquid crystal devices 12 and 14 and associatedretarders positioned in locations 44 ₂ and 44 ₃ is placed betweenorthogonally aligned polarizers 46 and 48, such that azimuthal direction40 of surface-contacting directors 18 c and 22 c of the respective ECBliquid crystal devices 12 and 14 make a 45° angle with the polarizationdirection of the incident light. For applied voltages of 1.41V, 1.74V,and 2.22V, the normalized transmitted luminance at normal incidence is75%, 50%, and 25%, respectively.

FIGS. 8B, 8C, and 8D show measured iso-transmitted luminance polarcontour diagrams of the above example of liquid crystal iris 10 of thefirst embodiment for applied voltages of 1.41V, 1.74V, and 2.22V, forwhich the normalized transmitted luminance at normal incidence is 75%,50%, and 25%, respectively. The central portions of these diagramsexhibit only a weak angular dependence of the transmission, therebyverifying the simulated results and showing the suitability of liquidcrystal iris 10 of the first embodiment for a camera iris application.

FIG. 9 shows a simulated transmitted luminance electro-optic curve of anexample of the second embodiment of the disclosed electro-optic liquidcrystal camera iris. The construction of the second embodiment is thesame as that of liquid crystal iris 10 shown in FIG. 5, except asdescribed below. The director fields of the liquid crystal devices ofthe second embodiment differ from the director fields 16 and 20 of ECBliquid crystal devices 12 and 14 of the first embodiment. The liquidcrystal devices and their associated director fields of the secondembodiment are, therefore, indicated by corresponding reference numeralsfollowed by primes. The liquid crystal MLC-7030 (Δn=0.1126) is used inthe simulation, and the cell gap of each of liquid crystal devices 12′and 14′ is set at 3.07 μm so that the product of cell gap, d, times thebirefringence of the liquid crystal, Δn, is approximately given by theformula Δn·d/λ=0.629, where λ is the design wavelength of 550 nm. Thepretilt angle is 3°. Liquid crystal device 12′ has a layer twist angleof 60° along the cell thickness dimension from alignment surface 30 ₁′to alignment surface 30 ₂′. Liquid crystal device 14′ has a layer twistangle −60° along the cell thickness dimension from alignment surface 38₁′ to alignment surface 38 ₂′. The twist angle of liquid crystal device14′ is of opposite twist or rotational sense to that of liquid crystaldevice 12′ because director field 20′ in liquid crystal device 14′ is amirror image of director field 16′ in liquid crystal device 12′. Liquidcrystal devices 12′ and 14′ are placed together so that azimuthaldirections 42′ of surface-contacting directors 18 c′ and 22 c′ at theadjoining or confronting surfaces of substrate plates 24 ₂′ and 32 ₁′ ofthe respective liquid crystal devices 12′ and 14′ are in parallelalignment. (Because of the 60°-twist angle, azimuthal directions 42′represent the projections of surface-contacting directors 18 c′ and 22c′ on the surfaces of substrate plates 24 ₂′ and 32 ₁′, respectively.)In the second embodiment, the input polarization direction is set toapproximately bisect the angular distance between azimuthal directions40′ and 42′ of surface-contacting directors 18 c at the respectivealignment surfaces 30 ₁′ and 30 ₂′ of liquid crystal device 12′. Thenormalized transmitted luminance is 50% for an applied voltage of 2.61V,and the normalized transmitted luminance at 6.63V is 0.1%, therebyresulting in a contrast ratio of 1,000. The second embodiment requiresno external retardation to achieve a high contrast ratio.

FIG. 10 shows the viewing angle dependence of the normalized transmittedluminance under application of a drive voltage of 2.61V, which produces50% normalized transmitted luminance at normal incidence. These data arepresented in FIG. 10 in the form of a normalized iso-transmittedluminance polar contour diagram. Comparing FIG. 10 with FIG. 2 for theprior art TN iris, it is apparent that there is remarkably less angularvariation in transmitted luminance. The liquid crystal iris of thesecond embodiment is eminently suitable for use as a camera iris becauseit can achieve a high contrast ratio and exhibits capability to maintaina uniform gray level over a wide range of light input angles.

FIG. 11 shows a simulated transmitted luminance electro-optic curve ofan example of a third embodiment of the disclosed electro-optic liquidcrystal camera iris. The construction of the third embodiment is thesame as that of liquid crystal iris 10 shown in FIG. 5, except asdescribed below. The director fields of the liquid crystal devices ofthe third embodiment differ from director fields 16 and 20 of ECB liquidcrystal devices 12 and 14 of the first embodiment. The liquid crystaldevices and their associated director fields of the third embodimentare, therefore, indicated by corresponding reference numerals followedby double primes. The liquid crystal MLC-7030 (Δn=0.1126) is used in thesimulation, and the cell gap of each of liquid crystal devices 12″ and14″ is set at 2.18 μm so that the product of cell gap, d, times thebirefringence of the liquid crystal, Δn, is approximately given by theformula Δn·d/λ=0.447, where λ is the design wavelength of 550 nm. Thepretilt angle is 3°. Liquid crystal device 12″ has a layer twist angleof 90° along the cell thickness dimension from alignment surface 30 ₁″to alignment surface 30 ₂″. Liquid crystal device 14″ has a layer twistangle −90° along the cell thickness dimension from alignment surface 38₁″ to alignment surface 38 ₂″. The twist angle of liquid crystal device14″ is of opposite twist sense to that of liquid crystal device 12″because director field 20″ of liquid crystal device 14″ is a mirrorimage of director field 16″ of liquid crystal device 12″. Liquid crystaldevices 12″ and 14″ are placed together so that azimuthal directions 42″of surface-contacting directors 18 c″ and 22 c″ at the adjoining orconfronting surfaces of substrate plates 24 ₂″ and 32 ₁″ of therespective liquid crystal devices 12″ and 14″ are in parallel alignment.(Because of the 90°-twist angle, azimuthal directions 42″ represent theprojections of surface-contacting directors 18 c″ and 22 c″ on thesurfaces of substrate plates 24 ₂″ and 32 ₁″, respectively.) In thethird embodiment, the input polarization direction is set toapproximately 20° with respect to azimuthal direction 40″ ofsurface-contacting directors 18 c″ at light input surface 52″ of liquidcrystal device 12″. The normalized transmitted luminance is 50% for anapplied voltage of 2.69V, and the normalized transmitted luminance at6.63V is 0.1%, thereby resulting in a contrast ratio of 1,000. The thirdembodiment requires no external retardation to achieve a high contractratio.

FIG. 12 shows the viewing angle dependence of the normalized transmittedluminance under application of a drive voltage of 2.69V, which gives 50%transmitted luminance at normal incidence. These data are presented inFIG. 12 in the form of a normalized iso-transmitted luminance polarcontour diagram. Comparing FIG. 12 with FIG. 2 for the prior art TNiris, it is apparent that there is remarkably improved uniformity of theangular dependence of the 50% gray level. The liquid crystal iris deviceof the third embodiment is eminently suitable for use as a camera irisbecause it can achieve a high contrast ratio and exhibits capability tomaintain a uniform gray level over a wide range of light input angles.

FIG. 13 is a block diagram of an example of a camera module 60 thatincludes the liquid crystal iris of any of the three embodiments of thedisclosed electro-optic liquid crystal camera iris. FIG. 13 shows liquidcrystal iris 10 of the first embodiment for purposes of convenience.Camera module 60 includes optical components of miniature sizes that canfit within a smart phone housing and provide full motion video and stillphotographs. Incoming light 50 entering camera module 60 is incident onliquid crystal iris 10, which controls the amount of light passingthrough it. A lens 62, which is illustrated as a single component butcould be a compound lens assembly, collects the light and focuses theimage onto an image sensor 64 of, for example, a CCD or CMOS type. Animage processor 66 processes the image data and sends image informationa controller 68, which exports the image to memory, a liquid crystaldisplay screen, or other storage or display medium. Controller 68 alsosends image information to an iris driver 70, which then applies theappropriate signals to liquid crystal iris 10 to control the amount oflight reaching image sensor 64. Control signals can also be applieddirectly to iris driver 70.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles thereof. For example, opticallytransparent electrodes 26 ₁ and 26 ₂ of liquid crystal device 12 andoptically transparent electrodes 34 ₁ and 34 ₂ of liquid crystal device14 can be patterned into concentric rings to produce an adjustable depthof field. This alternative is applicable also to the other embodimentsdescribed. Moreover, for liquid crystal device 12, the value of angle αmade by surface-contacting directors 18 c with alignment surface 30 ₁need not be equal to the value of angle α made by surface-contactingdirectors 18 c with alignment surface 30 ₂; and, for liquid crystaldevice 14, the value of angle α made by surface-contacting directors 22c with alignment surface 38 ₁ need not be equal to the value of angle αmade by surface-contacting directors 22 c with alignment surface 38 ₂.This alternative is applicable also to the other embodiments described.The scope of the present invention should, therefore, be determined onlyby the following claims.

The invention claimed is:
 1. A high-contrast electro-optic liquidcrystal camera iris providing angle independent transmission of incidentlight for uniform gray shades, comprising: first and second lightpolarizing filters; a first liquid crystal device including aspaced-apart pair of first electrode structures and a second liquidcrystal device including a spaced-apart pair of second electrodestructures, the first and second liquid crystal devices positionedbetween the first and second light polarizing filters and arranged inoptical series so that a surface of one of the first electrodestructures adjoins or confronts a surface of one of the second electrodestructures to form an interface between the first and second liquidcrystal devices, the interface characterized by substantially noselective polarization state blocking of incident light propagating fromone to the other of the adjoining or confronting surfaces of the firstand second electrode structures; the first liquid crystal device havingspaced-apart first alignment surfaces which are formed on interiorsurfaces of the first electrode structures and between which areconfined first liquid crystal directors, the first liquid crystaldirectors forming a first director field and including firstsurface-contacting directors that contact, and define an azimuthaldirection on, each of the first alignment surfaces; the second crystaldevice having spaced-apart second alignment surfaces which are formed oninterior surfaces of the second electrode structures and between whichare confined second liquid crystal directors, the second liquid crystaldirectors forming a second director field and including secondsurface-contacting directors that contact, and define an azimuthaldirection on, each of the second alignment surfaces; one of the firstand second director fields being a mirror image of the other of thefirst and second director fields; and the azimuthal directions definedon the first and second alignment surfaces formed on respective ones ofthe adjoined or confronting first and second electrode structures beingin parallel alignment.
 2. The liquid crystal camera iris of claim 1,further comprising one or more external retardation films positioned atone or more locations that include a location between the firstpolarizing filter and the first liquid crystal device, a locationbetween the first and second liquid crystal devices, and a locationbetween the second liquid crystal device and the second polarizingfilter.
 3. The liquid crystal camera iris of claim 1, in which the firstand second electrode structures are patterned into concentric rings toproduce an adjustable depth of field.
 4. The liquid crystal camera irisof claim 1, in which the first and second light polarizing filters havetransmission axes that are perpendicular to each other.
 5. The liquidcrystal camera iris of claim 1, in which the first and second liquidcrystal devices are homogeneously oriented electrically controlledbirefringence (ECB) devices containing liquid crystal with positivedielectric anisotropy.
 6. The liquid crystal camera iris of claim 1, inwhich the first and second liquid crystal devices arequasi-homeotropically oriented electrically controlled birefringence(ECB) devices containing liquid crystal with negative dielectricanisotropy.
 7. The liquid crystal camera iris of claim 1, in which thefirst and second liquid crystal devices contain liquid crystal withpositive dielectric anisotropy, and in which the first and second liquidcrystal director fields are of opposite rotational sense.
 8. The liquidcrystal camera iris of claim 7, in which the first and second liquidcrystal director fields of opposite rotational sense define twist anglesof about 60°.
 9. The liquid crystal camera iris of claim 8, in which thefirst liquid crystal device has a light input face on which incominglight having an input polarization direction is incident; in which thefirst surface-contacting directors define different azimuthal directionson different ones of the spaced-apart first alignment surfaces, thedifferent azimuthal directions thereby defining an angular distancebetween them; and in which the input polarization direction bisects theangular distance between the different azimuthal directions.
 10. Theliquid crystal camera iris of claim 7, in which the first and secondliquid crystal director fields of opposite rotational sense define twistangles of about 90°.
 11. The liquid crystal camera iris of claim 10, inwhich one of the first electrode structures of the first liquid crystaldevice has a light input face on which incoming light having an inputpolarization direction is incident, and in which the firstsurface-contacting directors contacting the first alignment surfaceformed on the interior surface of the one of the first electrodestructures define an azimuthal direction that makes an angle of about20° with the input polarization direction.
 12. The liquid crystal camerairis of claim 1, in which the first and second liquid crystal devicesare electrically controlled birefringent (ECB) devices that includerespective first and second liquid crystal cells, the first and secondliquid crystal cells having cell gaps that provide voltage-dependentphase shift changes of polarization components of the incident lightpropagating through the respective first and second liquid crystaldevices, and in which the phase shift changes imparted by the first andsecond liquid crystal devices add to provide light transmittance throughthe camera iris.